Technical Field
The present disclosure relates to an ultraviolet light-emitting diode (UV-LED) having improved efficiency, and a method for producing the same.
Description of the Related Art
Solid state light-emitting elements utilizing nitride semiconductor material, among others, blue light-emitting diodes have been widely used. Similar solid state light-emitting elements are in demand for application to a shorter wavelength range. Thus ultraviolet light-emitting diodes have been developed using similar material groups to that of the blue LEDs. In particular, LEDs for the deep ultraviolet range, or deep UV LEDs (DUVLEDs) have been under development, because ultraviolet (UV) light, to be more specific, light at a short wavelength of around 260 nm or in the deep ultraviolet range, is considered to have a variety of useful applications including sterilization, water purification, and medical applications. A typical DUVLED structure includes a sapphire substrate and has a layered structure made of gallium aluminum nitride series semiconductor, which contains aluminum (Al), gallium (Ga), and nitrogen (N) for its main composition. Output power of UV radiation for such DUVLEDs has been improved, and DUVLEDs operated at optical output level of 10 mW has been developed so far.
Technological challenges for such DUVLEDs include improvement of emission efficiency, as an example. The emission efficiency is measured by an external quantum efficiency ηEQE, which is defined as a ratio of optical energy emitted to outside of the LED to the input electric energy. The external quantum efficiency ηEQE can be expressed in a factorized form into three factors of an internal quantum efficiency ηIQE, an electron injection efficiency ηEIE, and a light extraction efficiency ηLEE. Thus, the following relationship is found among them: ηEQE=ηIQE×ηEIE×ηLEE.
Among the three factors the internal quantum efficiency ηIQE and the electron injection efficiency ηEIE have already been drastically improved due to continuing development efforts for the DUVLEDs so far. Specifically, technological solutions contributed to such improvement include reduction of crystalline dislocations in the emission layer for improving the internal quantum efficiency ηIQE (see, for example, Patent Document 1 and Non Patent Documents 1 and 2). For improving the electron injection efficiency ηEIE, it is effective to assist electron blocking performance in a p-type semiconductor layer by applying a structure called a multi-quantum barrier, or MQB, that utilizes a superlattice structure in the p-type semiconductor layer (see, for example, Non Patent Document 3). Currently a value of as much as 50-80% can be expected for a product of the internal quantum efficiency and the electronic injection efficiency ηIQE×ηEIE.